Localized two-dimensional shift correlated MR spectroscopy of human brain

Abstract

A two-dimensional (2D) chemical shift correlated MR spectroscopic (COSY) sequence integrated into a new volume localization technique (90°-180°-90°) for whole body MR Spectroscopy. Using the product operator formalism, a theoretical calculation of the volume localization as well as the coherence transfer efficiencies in 2D MRS is presented. A combination of different MRI transmit/receive rf coils is used. The cross peak intensities excited by the proposed 2D sequence are asymmetric with respect to the diagonal peaks. Localized COSY spectra of cerebral frontal and occipital gray/white matter regions in fifteen healthy controls are presented.

1. A method of whole body magnetic resonance (MR) spectroscopy utilizing localized chemical shift correlated magnetic resonance spectroscopic sequence (L-COSY), said L-COSY sequence comprising the steps of:
 a) applying a pulse train of at least three high frequency (rf) pulses to localize in one shot a volume of interest, a first slice selective 90° rf pulse, a second slice selective 180° rf pulse and a last slice selective 90° rf pulse, wherein the first and second slice selective rf pulses generate a first spin echo and the last slice selective 90° rf pulse generates a second echo that is a coherent transfer echo; 
 b) inserting an incremental period t 1  after the first spin echo and before the last slice selective 90° pulse; and 
 c) detecting a MR signal during interval t 2  after the last slice-selective 90° rf pulse and storing the MR signal data. 
 
   
   
     2. The method of  claim 1 , further comprising the steps of:
 d) attaching slice-selective Bo gradient pulses to the first slice selective 90° rf pulse, the second slice selective 180° rf pulse and the last 90° rf pulse; 
 e) applying slice refocusing Bo gradient pulses after the first slice selective 90° rf pulse, before the last 90° rf pulse and after the last 90° rf pulse; 
 f) applying Bo gradient crusher pulses before and after the second slice selective 180° rf pulse and before and after the last 90° rf pulse. 
 
   
   
     3. The method of  claim 2 , further comprising
 g) repeating steps a) to f) with different values of t 1;  and 
 h) subjecting the stored signal data to a double Fourier transformation with respect to t 1  and t 2  to obtain a two dimensional (2D) MR spectrum. 
 
   
   
     4. The method of  claim 3 , further comprising suppressing MR signals of a solvent. 
   
   
     5. The method of  claim 4  wherein the solvent is water. 
   
   
     6. A non-invasive method for identifying brain metabolites comprising the steps of:
 a) subjecting a volume of interest to the L-COSY sequence of  claim 3 , wherein the volume of interest is localized within a region of the brain; 
 b) identifying at least one characteristic cross peak corresponding to a brain metabolite within the 2D MR spectrum. 
 
   
   
     7. A method according to  claim 6 , wherein the characteristic cross peak is asymmetric with respect to the diagonal peaks. 
   
   
     8. The method of  claim 6 , wherein the characteristic cross peak corresponds to a metabolite selected from the group consisting of N-acetyl aspartate (NAA), glutamate/glutamine (Gix), myo-inositol (ml), creatine (Cr), choline (Ch), aspartate (Asp), γ-aminobutyrate (GABA), threonine (Thr), glutathione (GSH) and macromolecules (MM). 
   
   
     9. The method of  claim 6  wherein the region of the brain is frontal gray/white matter or occipital gray/white matter. 
   
   
     10. The method of claim of  6 , further comprising extracting a cross-sectional one dimensional (1 D) MR spectra of a metabolite from the 2D L-COSY spectrum. 
   
   
     11. A method of volume localization performed on a magnetic resonance imaging (MRI) scanner utilizing a coherence transfer based spin-echo spectroscopy (CABINET) sequence, said CABINET sequence comprising the steps of:
 a) applying a pulse train of at least three high frequency (rf) pulses to localize in one shot a volume of interest, a first slice selective 90° rf pulse, a second slice selective 180° rf pulse and a last slice selective 90° rf pulse, wherein the first and second slice selective if pulses generate a first spin echo and the last slice selective 90° rf pulse generates a second echo that is a coherent transfer echo; 
 b) inserting an incremental period t 1  after the first spin echo and before the last slice selective 90° pulse; and 
 c) detecting a MR signal during interval t 2  after the last slice-selective 90° rf pulse and storing the MR signal data.